This invention relates to a nonaqueous electrolytic solution cell with excellent low-temperature characteristics, long-term stability and high energy density.
Along with recent trends in weight and size reductions of electric appliances, attention to a lithium cell has been growing due to its high energy density. And along with diversification of lithium cell applications, it has been needed to improve the cell characteristics.
As solvent for an electrolyte of the lithium cell, widely used is nonaqueous organic solvent including carbonates such as, for example, ethylene carbonate, propylene carbonate, diethyl carbonate or xcex3-butyrolactone; and esters.
Among these, propylene carbonate has an excellent property as a main solvent of the electrolyte, since it has a high dielectric constant, well dissolves lithium salts as a solute (electrolyte), and shows a high electric conductivity even at low temperatures. Independent use of propylene carbonate may, however, excessively increase viscosity of the electrolyte, which may degrade the discharging characteristics in particular at low temperatures. While a mixed solvent of propylene carbonate and 1,2-dimethoxyethane has been proposed, the long-term stability and safety still remain in problem due to a relatively low boiling point 1,2-dimethoxyethane.
As for secondary battery using propylene carbonate, a problem of gas generation may arise depending on types of the electrode materials. For example, it is known that, when a variety of graphitic electrode materials is used singly, or mixed material of graphitic electrode material and an electrode material capable of liberating lithium is used for a negative electrode, propylene carbonate is vigorously decomposed on the surface of the graphitic electrode, thereby to prevent lithium from being smoothly occluded into or liberated from the graphitic electrode (7th International Symposium on Li Batteries, p.259, 1995).
Thus use of ethylene carbonate as a solvent for the electrolytic solution came into recent trend, since it is less causative of such decomposition reaction. Ethylene carbonate has a solidification point (36.4xc2x0 C.) higher than that of propylene carbonate, so that it is not used singly but in combination with low-viscosity solvents which include dialkyl carbonates such as dimethyl carbonate and diethyl carbonate; dimethoxyethane; and dioxolane (xe2x80x9cKino Zairyo (Functional Materials)xe2x80x9d, Vol.15, April, p.48, 1995). The low-viscosity solvents are, however, generally low in boiling points, which raises apprehension that inner pressure of the cell will increase when they are added in a large amount, thereby to degrade safety due to solvent leakage. Problems on solidification of the electrolytic solution and a low electric conductivity under low temperatures are also frequently encountered. Although a mixed solution of ethylene carbonate and diethyl carbonate or the like is used as an electrolytic solution for lithium secondary batteries in such situation, such batteries still suffer from insufficient cycle characteristics.
To solve these problems, it has been proposed to use sulfite compounds as the solvent (e.g., Japanese Unexamined Patent Publications No. 06-302336, No. 07-122295, No. 08-96851 and No. 09-120837). In these publications, it is reported that the electrolytic solution using sulfite compounds are high in electric conductivity and low in viscosity, and thus exhibits excellent cell characteristics at low temperatures. It has also been proposed to use sulfolane compounds as the solvent in terms of improving cycle characteristics of secondary batteries (e.g., Japanese Unexamined Patent Publication No. 03-152879).
Malfunctions of the batteries have, however, been found out when the sulfite compounds or sulfolane compounds, each having an Sxe2x80x94O bond, are used for the electrolytic solution. It is, in particular, demonstrated as a marked degradation in the cycle characteristics of secondary batteries, and there is a room for further improvements before the practical use.
Referring to such situation of the conventional technologies, it is therefore an object of the present invention to select preferable compounds as a solvent for nonaqueous electrolytic solution and find out conditions under which the functions of the solvent is fully exerted in the cell. More specifically, it is an object of the present invention to provide a nonaqueous electrolytic solution cell excellent in low-temperature characteristics and long-term stability, and also in cycle characteristics particularly for use as a secondary battery.
After extensive investigations to achieve such objects, the inventors of the present invention found out that a nonaqueous electrolytic solution cell with quite excellent characteristics can be obtained by properly selecting, as a solvent for the nonaqueous electrolytic solution, specific compounds having Sxe2x80x94O bonds, and by properly determining materials for composing the current collector and outer can with which the electrolytic solution will contact.
Thus the present invention provides a nonaqueous electrolytic solution cell comprising a negative electrode containing lithium as an active material, a positive electrode, a nonaqueous electrolytic solution consisting of solute and organic solvent, a separator, and an outer can; the organic solvent containing at least one of compounds represented by the formula (1):
R1xe2x80x94Axe2x80x94R2xe2x80x83xe2x80x83(1)
(in which, R1 and R2 independently represent an alkyl group which may be substituted with an aryl group or halogen atom; an aryl group which may be substituted with an alkyl group or halogen atom; or may be taken together to form, together with xe2x80x94Axe2x80x94, a cyclic structure which may contain an unsaturated bond, where xe2x80x9cAxe2x80x9d being represented by any one of the formulae (2) to (5)): 
and, a current collector for the positive electrode and an electrolytic solution-contacting portion on the positive electrode side of the outer can being made of valve metal or alloy thereof.
Compounds represented by the formula (1) are exemplified as ethylene sulfite, dimethyl sulfite, sulfolane, sulfolene and 1,3-propane sultone. A preferable example of the organic solvent can be a mixture of ethylene sulfite and propylene carbonate. Examples of the electrolyte are LiClO4, LiPF6, LiBF4, LiCF3SO3, LiN(CF3SO2)2, LiN(CF3CF2SO2)2, LiN(CF3SO2) (C4F9SO2) and LiC(CF3SO2)3. The concentration of the solute is preferably about 0.5 to 2.0 mol/litter.
The valve metal and alloy thereof used in the present invention are preferably Al, Ti, Zr, Hf, Nb, Ta or alloys containing these metals, where Al or Al alloy is more preferable. Materials for the negative electrode can be selected from carbonaceous materials such as graphite capable of occluding and liberating lithium; metal oxide materials capable of occluding and liberating lithium; lithium metal; and lithium alloy. In particular, preferably used is a carbonaceous material having a xe2x80x9cdxe2x80x9d value for lattice plane (002) in X-ray diffractometry of 0.335 to 0.37 nm, more preferably 0.335 to 0.34 nm. Materials for the positive electrode can be selected from lithium-transition metal composite oxide material capable of occluding and liberating lithium; transition metal oxide material; and carbonaceous material.
For the case that the cell of the present invention is used as a secondary battery, it is preferable to apply current pulses at the initial charging. The applied pulses are preferably of rectangular wave with a constant current value of 0.01 to 100 A per one gram of the active material for the negative electrode, a pulse width of 0.01 to 300 seconds, a pulse separation of 0.1 to 300 seconds, and a pulse count of 10 to 1000 counts.